The invention relates to a laser utilizing a negative branch type unstable cavity resonator, and is especially suitable for high energy applications.
In essence, the prior art embraces two main designs of cavity resonator utilized for high energy laser oscillation systems in conjunction with high gain active laser media (as used, for instance, with Nd:YAG or CO.sub.2 sources).
In a first design, use is made of a conventional stable resonator having one concave mirror and one flat mirror, in which a system of lenses is located.
The lenses together form a an inverse telescope the function of which is to increase the transverse dimension of the laser beam in one of the two subsections of the resonator (these being created in effect by embodiment of the telescope) in which the active laser medium is located.
The beam may be out-coupled by way of one of the mirrors, which would be partially transmitting to admit the passage of laser radiation.
In a resonator of this type, there is a risk of damage to the optics in the smaller section of the resonator, where the laser beam gets its smaller dimension, and the alignment of the resonator becomes critical by reason of the inclusion of two extra optical components.
In a second design of high energy laser resonator, use is made of a positive branch unstable cavity. The most common embodiment of such a cavity resonator consists of two mirrors, one convex and one concave, aligned frontally along an optic axis and confocally disposed.
There are several methods of embodying this basic design, methods which differ mainly in the different techniques adopted for out-coupling the laser beam.
Whatever the particular out-coupling technique is ultimately adopted, the essential feature of the positive branch unstable cavity resonator is that the field of radiation within the cavity always stays, during repeated reflection between the two mirrors, in `near field` conditions. This gives rise to the disadvantage that the beam is respectively chipped by cavity apertures at not negligible intensity levels.
The result is that diffraction fringes of a certain amplitude develop, and the intensity profile of the beam does not appear uniform, but modulated at high spatial frequency. An operating characteristic such as this leads to poor performance in terms of propagation and focusing of the laser beam.
The use of negative branch type unstable cavity resonators in high energy laser emission has not been widespread thus far by reason of the difficulties encountered in eliminating hazards connected with focusing of the laser beam on a focal point located within the cavity resonator. Given the high intensity of a focused laser beam, deterioration or destruction of one or more optical components may result, and in any case, there may be serious limitations imposed on performance of the laser in terms of output.
The object of the invention is that of overcoming the disadvantages and shortcomings described above.